Labile systems substitution reactions

Ligand substitution reactions of NO leading to metal-nitrosyl bond formation were first quantitatively studied for metalloporphyrins, (M(Por)), and heme proteins a few decades ago (20), and have been the subject of a recent review (20d). Despite the large volume of work, systematic mechanistic studies have been limited. As with the Rum(salen) complexes discussed above, photoexcitation of met allop or phyr in nitrosyls results in labilization of NO. In such studies, laser flash photolysis is used to labilize NO from a M(Por)(NO) precursor, and subsequent relaxation of the non-steady state system back to equilibrium (Eq. (9)) is monitored spectroscopically. 

Some kinetic data on the trans effect are now available. More detailed systematic studies are needed, but quantitative information does provide the magnitude of the trans labilizing ability of various ligands for substitution reactions in these systems. For complexes of the type [PtNH3LCl2]", where the leaving Cl" is trans to L, the trans effect order of L is approximately (28),... 

Redox substitution reactions can be photoinitiated. Taube first proposed that the photo-catalyzed substitution of PtCll- occurs by an electron-transfer process (equation 560) to give a kinetically labile platinum(III) intermediate.2040 Further work on this system has shown that the exchange occurs with quantum yields up to 1000,2041-2043 and the intermediate has beer assigned a lifetime in the fis range.2044 Recently the binuclear platinum(III) complexes Pt2(P2OsH2)4Xr (X = Cl, Br, I) have been found to show similar behavior and both photoreduction and complementary redox reactions are again proposed to explain the substitution behavior.1500... 

In contrast to the 18-electron systems, the 17-electron V(CO)6 is substitutionally labile. It is one of the few 17-electron systems that can be studied under non-transient conditions but there is considerable evidence that the conclusions drawn from the study of this system can be applied to transient 17-electron species such as Mn(CO)5. Data from a published study of CO exchange with V(CO)6 are absent but there is much anecdotal reporting. For example, a t/2 value of 7 h for the reaction in heptane at 10 °C under 2 atm of CO is quoted.97 The substitution reactions of V(CO)6 are associatively activated (k2 for the reaction between V(CO)6 and PPh3 in hexane at 25 °C =... 

The substitution reaction in which a molecule of solvent replaces one of the ligands represents one of the most commonly and conveniently studied processes in coordination chemistry. In labile systems, analysis of the relaxation kinetics in the complex formation studies will, of course, give the rate constants for the solvolysis as well as those for the complex formation. In inert systems... 

Benzyloxy(cyano)carbene would be expected to be electrophilic by virtue of the calculated selectivity index, Wp -HjOccN 111- This has not yet been experimentally proved as 3-bcnzyloxy-3-cyano-3//-diazirine is rather labile and cannot be isolated if it is synthesized by the substitution of chlorine by a cyano group in 3-benzyloxy-3-chloro-3if-diazirine consequently, this substitution reaction is carried out in the presence of an alkene, hence, preparing 1-benzyloxy-cyclopropane-l-carbonitrile derivatives. However, the cyanide ion present in the system induces the polymerization of electrophilic alkenes, such as acrylonitrile or methyl acrylate. 

Substitution reactions are well documented for the three Group 6 metal hexacarbonyls. Since they are low-spin d systems, reactivity is normally lower than that of other metal carbonyls. To promote substitution reactions, it is necessary to use elevated temperatures or uv irradiation. The hexacarbonyls usually undergo substitution via a dissociative mechanism, unless direct attack of a nucleophile at coordinated carbonyl groups occurs. Usually MoCCO) is more reactive than its two congeners. This is shown, inter alia, by kinetic studies on the isotopic exchange reaction between M( CO)6 and The higher kinetic lability of Mo(CO)6 may effect some of the reductions and oxidations, when the electron transfer process is accompanied by loss of carbonyl groups, e.g., equations (a), (e)-(g), and (1). 

The development of fast reaction techniques (see Chap. 2, Vol. 1, and ref. 1) has brought about, over the last ten years, a detailed (but still incomplete) knowledge of the substitution reactions of labile metal ions. Successful measurement of the rates of these and other very rapid reactions, previously inaccessible to investigation, has meant a complete reappraisal of the term instantaneous as applied to reacting chemical systems. The subject has been reviewed a number of times previously, notably by Eigen [2], Eigen and Wilkins [3], Sutin [4], and Diebler et al. 

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